The adhesive and waterproofing properties of asphalt have been known for centuries. Of the many applications of asphalt today, most asphalt derived from petroleum is used as paving material, commonly called asphalt cement. Asphalt (bitumen) is mixed with aggregate (rock) for use in paving.
Asphalt cement compositions must meet certain performance criteria or specifications to be considered useful for road paving. State and federal agencies issue specifications for various asphalt applications including specifications for use as road pavement. Performance standards are set forth in various standards of the American Society for Testing and Materials (ASTM) and the American Association of State Highway and Transportation Officials (AASHTO). Current Federal Highway Administration specifications designate an asphalt as meeting defined parameters relating to properties such as performance grading, viscosity, toughness, tenacity, and elastic recovery. Each of these parameters defines an important feature of an asphalt composition. Compositions failing to meet one or more of these parameters may be unacceptable for use as road pavement material.
More specifically, asphalt cement is viscoelastic in nature and its behavior is dependent on both temperature and loading. In hot conditions, asphalt cement acts as a viscous liquid and is subject to viscous flow. Under these conditions, less viscous hot mix asphalt pavements flow under repeated wheel loading to form ruts. Such permanent deformation is called rutting. In cold climates or under rapidly applied loads, asphalt cement behaves like an elastic solid. If stressed beyond the material's capacity, such elastic solids may break. Some asphalt cements may become too brittle and crack when excessively loaded, resulting in low temperature cracking. Neither of these conditions is desirable in an asphalt cement.
Conventional asphalt compositions frequently cannot meet all of the requisite specifications, and damage to the resulting road can occur, such as permanent deformation, thermally induced cracking, and fatigue cracking. Conventional asphalts can be modified, however, with other substances to improve their performance properties. A wide variety of polymers have been used in this regard.
Polymer modified asphalt (“PMA”) has become common in road paving and roofing and may represent as much as 20% of all asphalt used today. Improvements in rutting resistance, thermal cracking, fatigue damage, stripping, and temperature susceptibility have led polymer modified binder to be substituted for asphalt in many paving applications, including hot mix, cold mix, chip seals, hot and cold crack filling, patching, and slurry seal. PMAs are used wherever performance and durability are desired. Asphalt specifiers are finding that many of the Superpave binder grades need polymer modification to meet all the requirements for high temperature rutting resistance and thermal cracking resistance at low temperatures. Superpave, which stands for Superior Performing Asphalt Pavements, represents an improved, standardized system for specifying, testing, and designing asphalt materials.
Common polymers used for asphalt modification include styrene-butadiene block copolymers, styrene-isoprene block copolymers, ethylene-vinyl/acetate copolymers, polyethylene, and polypropylene. Typically, polymer addition involves addition of solid polymer, possibly after grinding, to a rear-shear mixing vessel containing asphalt generally heated above 325° F. for a period of time to assure thorough mixing. This tends to be a labor and capital intensive process.
Despite the benefits of adding polymers to asphalt to improve physical and mechanical performance, polymers alone may not optimize asphalt performance. Also, the cost of adding polymer to the asphalt at levels sufficiently high to meet desired specifications may be prohibitive. As a result, the industry has developed chemical agents that enhance the performance of the polymer modifiers. Many of these agents have been termed crosslinkers and are widely used in the industry. They are believed to either crosslink the polymer to the asphaltene component of the asphalt or crosslink the polymer and improve properties. Improvements include superior performance in penetration, ductility, phase angle tests, as well as increased Strategic Highway Research Program (SHRP) grading. Use of chemical agents can result in more efficient utilization of the polymer, thus reducing the required level of polymer loading. There also may be improved compatibility between the polymer and the asphalt resulting in less separation of the polymer.
The earliest crosslinking systems involved use of sulfur. U.S. Pat. No. 4,130,516 discloses an asphalt polymer composition obtained by hot-blending asphalt with 3% to 7% by weight of elemental sulfur and 0.5% to 1.5% by weight of a natural or synthetic rubber. U.S. Pat. No. 3,803,066 discloses a process for preparing a rubber modified asphalt by blending the rubber in amounts up to 10% by weight, and blending into the mix an amount of sulfur such that the weight ratio of sulfur to rubber is between 0.3 and 0.9. A catalytic quantity of a free radical vulcanization accelerator is then added to effect vulcanization.
A more recent patent involving sulfur is U.S. Pat. No. 6,180,697, which describes the use of a thermoplastic elastomer in concert with crosslinking formulations that include elemental sulfur, zinc-2-mercaptobenzothiazole, zinc oxide, and dithiodimorpholine. U.S. Pat. No. 6,407,152 describes a crosslinker including two components. One component is a thiopolymer prepared by the reaction of butyl cresol and sulfur dichloride and the other component may be selected from the group consisting of elemental sulfur, polythimorpholine, zinc-2-mercaptothiazole, and mixtures thereof.
U.S. Pat. No. 6,057,390 describes a method for improved high temperature performance of asphalt comprising addition of a crosslinkable polymer (i.e., SBS triblock copolymer) and addition of dioxime of 1,4-benzoquinone or derivatives and optionally free radical initiators (i.e., organic peroxides). Another crosslinker for rubber modified asphalt compositions is provided by U.S. Pat. No. 6,486,236. The rubber includes at least a polydiene and may include vinyl-substituted aromatic monomer units. The rubber is cured by a bismaleimide to improve softening points of the asphalt.
Asphalt has also been modified by the addition of polymerizabie monomers to impact physical properties of the asphalt. A process is taught by U.S. Pat. No. 3,547,850 for producing polymer-asphalt compositions that have a high softening point and improved penetration ratio, by effecting a polymerization in hot asphalt. This patent claims addition of an alkali metal (as a catalyst or initiator) and a monomer selected from the group consisting of conjugated dienes containing 4–12 carbons and vinyl substituted aromatic compounds, in heated asphalt to effect the polymerization of used monomers therein.
Another modified system has been described in U.S. Pat. No. 4,273,685 to aid the compounding of asphalt systems with fillers such as glass fibers. This describes asphalt that has been reacted with a polymerizable vinyl monomer and a rubbery polymer added separately. The reaction is carried out at a temperature at which the vinyl aromatic monomer and the rubbery polymer react with the asphalt. Improved bonding of chemical modified asphalts with reinforcing fillers was also noted in U.S. Pat. No. 4,332,705, which claims asphalt reacted with a polymerizable monofunctional vinyl aromatic monomer such as styrene, a polyfunctional polymerizable vinyl aromatic monomer such as divinylbenzene, and a rubbery polymer added separately.
Yet another factor for consideration in asphalt compositions is the use of solvents. Solvents have been used to fluidize asphalt polymer compositions, such as with small amounts of sulfur, as in U.S. Pat. No. 4,242,246. Such solvents also may be used to fluidize polymers. The use of large amounts of solvents, however, may lower the viscosity of the resulting asphalt compositions such that they become too soft for road paving applications. In contrast, the incorporation of solid rubbers directly into asphalt presents process and handling difficulties and the uniformity of the final composition is often difficult to achieve-or control. Separation of the solid polymer from the asphalt is also problematic. It is therefore desirable to develop compositions and methods for adding polymers to asphalt in a reactive monomer solvent in order to improve processing, achieve and sustain compositional uniformity, while improving or at least not compromising requisite asphalt properties.